Ok, I don't wanna sound naive, I know Mark's not perfect, nobody is, but... is there any chance Rudy lied to him about who built the guitar? This whole thing is so weird. I see three possible scenarios here:
1. John is telling the truth and for some reason Mark is favoring Rudy;
2. John is lying, Rudy built it just like it was shown in Guitar Stories;
3. Rudy lied to Mark and Mark believes he built it.
I'm inclined to believe the first option is the truth...